A typical inkjet printer uses one or more printheads to form an ink image on an image receiving surface. Each printhead typically contains an array of individual inkjets for ejecting drops of ink across an open gap to an image receiving surface to form an image. The image receiving surface may be the surface of a continuous web of recording media, a series of media sheets, or the surface of a rotating image receiving member, such as a print drum or endless belt. Images printed on a rotating image receiving member are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, frequency, and/or duration of the signals affect the amount of ink ejected in each drop. The firing signal is generated by a printhead controller with reference to digital image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member with reference to the digital image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and in the colors represented by the image data, the printheads must be registered with reference to the imaging surface and with the other printheads in the printer. While existing techniques can be used to detect errors in the placement of ink drops on the image receiving member, the correction of ink drop placement errors can present challenges. To correct process direction errors, the printer adjusts a timing offset used to control when firing signals are delivered to particular inkjets. In an existing printer, the inkjets in a printhead operate in a synchronous manner at a predetermined frequency. During each cycle of the frequency, an inkjet can either eject an ink drop in response to receiving an electrical firing signal from a controller, or not eject an ink drop when the controller does not deliver a firing signal. The resolution of images printed by the inkjet in the process direction is affected by the predetermined frequency and the velocity of the image receiving member. For example, if the printhead is operated with a frequency of 13 KHz, then an inkjet can eject up to 13,000 ink drops per second. If the image receiving member moves past the inkjet at a rate of approximately 37.14 inches per second, then the inkjet can form a line of ink drops in the process direction with a resolution of 350 drops per inch, where each drop lands to form a pixel on the image receiving surface.
Inkjets do not always operate flawlessly. The trajectory of ink drops ejected from an inkjet do not always fly true from the aperture to the image receiving surface. In fact, the paths traveled by ink drops ejected by an inkjet vary with the frequency at which the inkjet is fired, the frequency, duration, and/or amplitude of the firing signal that activates the actuator, the number of cycles that the inkjet has been inactive before the inkjet is activated, as well as other factors. Known printers operate inkjets to form test patterns on image receiving surfaces, generate image data of those patterns on the surface, and analyze those patterns to quantify errors in the position of the ejected ink drops, particularly first ink drops ejected after a relative period of inactivity and last drops ejected in sequence of contiguous inkjet firings. Once these errors are quantified, a controller can delay or expedite the delivery of a firing signal to alter the location where an ink drop lands in the process direction. These adjustments, however, can be no finer than a single pixel. That is, the smallest adjustment is either to operate the inkjet as if to cause the inkjet to eject the ink drop on the preceding row with the result that the ink drop is closer to the intended position due to the quantified error or to operate the inkjet as if to cause the inkjet to eject the ink drop on the next row with the result that the ink drop is closer to the intended position due to the quantified error in the opposite direction. While such an adjustment can help correct larger errors in the placement of ink drops, drop placement errors may still be noticeable. For example, an adjustment of one pixel in the process direction can overcompensate for an identified error and produce a new error.
One solution to improve the precision of ink drop placement is simply to operate the inkjets in the printhead at a much higher frequency for higher-resolution printing that enables finer compensation of drop placement errors. The operating characteristics of many printheads, however, render this solution impractical for many printers. For example, various fluidic, mechanical and physical characteristic of the inkjets in a given printhead mean that the individual inkjets can generally only be fired at a given maximum frequency. At rates greater than this maximum frequency, some of the inkjets in the printhead begin misfiring or producing inconsistencies in ink drop size and placement. Additionally most printheads synchronize the operation of the inkjets with an external trigger signal instead of operating the individual inkjets independently. In light of the operational limitations of printheads, improvements to the operation of inkjet printers to reduce errors in ink drop placement would be beneficial.